The apparatus as defined in the foregoing is known from European Patent application EP 671,739 A2, corresponding to U.S. Pat. No. 5,644,582, document D1 in the list of related documents. The apparatus is in the form of an apparatus for recording the digital information signal on a record carrier, such as a magnetic record carrier, and comprises error correction encoding means and channel encoding means.
A convenient definition of channel coding is: the technique of realizing high transmission reliability despite shortcomings of the channel, while making efficient use of the channel capacity. In essence, information theory asserts that a stationary channel can be made arbitrarily reliable given that a fixed fraction of the channel is used for redundancy.
In transmission and recording/reproduction systems, source data is commonly translated in two successive steps: via (a) error-correction code and (b) channel (or modulation) code, see document D4 in the list of related documents.
Error-correction control is realized by adding extra symbols to the conveyed message. These extra symbols make it possible for the receiver to detect and/or correct some of the errors that may occur in the received message.
There are many different families of error-correcting codes. Of major importance for recording applications is the family of Reed-Solomon codes (RS). The reason for their pre-eminence in e.g. recording/reproduction systems is that they can combat combinations of random as well as burst errors. By exploiting the redundancy present in the retrieved signal, the decoder reconstitutes the input sequences as accurately as possible.
A channel encoder has the task of translating arbitrary user information plus error-correction symbols into a sequence that complies with the given channel constraints. Examples are spectral constraints or run-length constraints. The maximum information rate given the channel input constraints, is often called the Shannon capacity of the input-constrained noiseless channel. A good code embodiment realizes a code rate that is close to the Shannon capacity of the constrained sequences, uses a simple implementation, and avoids the propagation of errors at the decoding process, or, more realistically, a code with a compromise between these competing attributes.
Current recording code implementations are very often byte-oriented. The efficiency of such codes, in terms of channel capacity, is typically less than 95%. In accordance with the adage "The bigger the better", a greater code efficiency can only be realized by utilizing codes with very long codewords of typically 500-1000 bits. Although we know how to construct such efficient codes in theory, the key obstacle to practically approaching channel capacity is the massive hardware required for encoding and decoding, as hardware grows with the number of codewords used, i.e., exponentially with the codeword length.
The use of algebraic coding techniques, such as enumeration, makes it possible to implement codes whose complexity grows polynomially with the codeword length. The algebraic coding technique makes it possible to translate source words into codewords and vice versa by invoking an algorithm rather than performing the translation with a look-up table. Reference is made in this respect to the not yet published International Patent Application No. WO 96/00045, document D3 in the list of related documents.
A second drawback of the use of long codewords, however, is the risk of extreme error propagation. Single channel bit errors may result in error propagation which destroys the entire decoded word, and, of course, the longer the codeword the greater the number of symbols affected.
The above citations are hereby incorporated in whole by reference.